In using a spectrophotometer, it is usually necessary to disperse the incident light beam into a plurality of different wavelengths or wavelength intervals in order to fully analyze the content of the incident light beam. One means of such dispersion is disclosed in U.S. Pat. No. 4,678,332, issued to Rock et al., in which a fiberoptics reformatter having a plurality of fibers is used for dispersion of the incident beam into its component wavelengths.
Another means of wavelength dispersion is disclosed in U.S. Pat. No. 4,681,445, issued to Perkins, in which a plurality of beam splitters, each functioning in a different part of the wavelength spectrum, is used to provide wavelength dispersion of an incident light beam. Crane, in U.S. Pat. No. 4,743,114, discloses the use of Fabry-Perot interferometer scanning using a nutating etalon in which the incidence angle of the light beam relative to the interferometer is caused to vary periodically in two perpendicular directions of rotation.
Use of a blazed diffraction grating or similar means of wavelength dispersion is disclosed in U.S. Pat. No. 4,758,090, issued to Schuma, and in U.S. Pat. No. 4,718,764, issued to Fink, and in U.S. Pat. No. 4,776,696, issued to Hettrick et al. In U.S. Pat. No. 4,731,881, Geller discloses means for blocking all light of the incident beam except a narrow pass band of wavelengths whose wavelength versus intensity content is then analyzed.
FIG. 1 illustrates a simple Fabry-Perot etalon 11, used as a narrow band pass filter in the prior art. An incident wave 12 arrives at a first surface 13 of the etalon that is partly reflecting and partly transmitting. A transmitted portion 14 of the light beam passes through the first reflecting surface 13 and propagates to a second parallel surface 15 that is partly reflecting and partly transmitting. The transmitted portion 14 of the light beam 12 is reflected back and forth between the reflecting surfaces 13 and 15 many times, with a portion of this light being transmitted through the second reflecting surface 15 at each pass. This produces a transmitted light beam 16 whose amplitude manifests well-known interference effects associated with the etalon 11 and depending upon the (constant) distance of separation h between the two reflecting planes 13 and 15. In this simplest approach to the Fabry-Perot etalon, the thickness of the reflecting surfaces 13 and 15 is ignored so that the only interference effect comes from the distance of separation of the surfaces 13 and 15. The distance of separation h is normally chosen to be .lambda. sec .THETA./2n, where n is the refractive index of the material between the two reflecting surfaces 13 and 15 and .THETA. is the incidence angle of the light received at the first reflecting surface.
Another narrow band pass filter discussed in the prior art is a thin film edge filter, which consists of a thin film of material that has a transition edge at a predetermined range of wavelengths .lambda.=.lambda..sub.0, is substantially fully transparent for wavelengths .lambda.&gt;.lambda..sub.0 and is substantially reflective for a continuous band of wavelengths .lambda.&lt;.lambda..sub.0. This filter is discussed by H.A. McLeod in Thin-Film Optical Filters, MacMillan, 1986, pp. 188-233, incorporated by reference herein, and one embodiment 17 is illustrated in FIG. 2. In FIG. 2, a first sequence of identical thin films 19 of quarter wave thickness .lambda./4n.sub.H of material 25 of high refractive index n.sub.H alternates with a second sequence of identical thin films 21 of quarter wave thickness .lambda./4n.sub.L of material of lower refractive index n.sub.L. An incident light beam 22 having a plurality of wavelengths approaches the edge filter 17, and after the beam is reflected and transmitted at various interfaces of the filter, a light beam 23 emerges from the filter 17 with long wavelength components .lambda.&gt;.lambda..sub.0 attenuated minimally, with wavelength components in a range .lambda..sub.1 &lt;.lambda.&lt;.lambda..sub.0 attenuated (by reflection, or alternatively by transmission) almost completely, and with shorter wavelength components .lambda.&lt;.lambda..sub.1 attenuated moderately or not at all. For a choice of germanium (n.sub.H =4.0) and silicon monoxide (n.sub.L =1.70) as the thin film materials (13 layers each) and a reference wavelength .lambda..sub.0 =4.0 .mu.m, the computed optical transmittance exhibits a totally reflective band between .lambda.= 3.3 .mu.m and .lambda.=5.4 .mu.m, as illustrated by McLeod, op.cit., in FIG. 6.2 on page 190. The configuration of FIG. 2 may also be used as a multilayer thin film to provide a filter that is highly transmissive or highly reflective in a specified wavelength band. Other suitable choices of filter material include quartz (n.sub.L =1.45), MgF.sub.2 (n.sub.L =1.38), Na.sub.3 AlF.sub.6 (n.sub.L =1.35), PbCl.sub.2 (n.sub.H =2.20) and ZnS (n.sub.H =2.35).
In a more sophisticated approach, the thickness of one of the reflecting surfaces 13 and 15 in FIG. 1 might also be taken into account so that additional interference effects arise from reflections at the two spaced-apart boundary surfaces of the reflecting surface that has non-zero thickness. This leads to consideration of a compound Fabry-Perot etalon that has two sets of reflecting surfaces and, usually, different distances of separation between each of the pairs of reflecting surfaces. The compound Fabry-Perot interferometer is discussed briefly in E.U. Condon and H. Odishaw, Handbook of Physics, McGraw-Hill, 1967, pp. 6-100 to 6-102 and in F.A. Jenkins and H.E. White, Fundamentals of Optics, McGraw-Hill, 1976, pp. 301-303.
At the same time, it may be desirable to alter the intensity distribution of light received at one or more detectors associated with the spectrophotometer or colorimeter to compensate for other processes. Young, in U.S. Pat. No. 4,740,082, discloses a spectrophotometer in which a photosensitive detector is uniformly illuminated for all areas of an aperture through which a portion of the light beam passes. In U.S. Pat. No. 4,264,211, Biggs discloses a light sensor in which the incident radiation received is corrected by the cosine law that applies to radiation received at a non-zero incidence angle by a radiation conductor.
A variable color filter is disclosed by Illsley et al. in U.S. Pat. No. 3,442,572, using a wavelength filter positioned on the circumference of a large circle, where the filter thickness increases linearly with increase of the azimuthal angle .THETA. (0&lt;.THETA.&lt;2.pi.) of the position on the circle circumference. This invention relies on a fabrication method and apparatus, disclosed and claimed in two division U.S. Pat. Nos. 3,530,824 and 3,617,331, that probably cannot be used to fabricate a filter whose thickness is not linearly increasing with increase in a spatial coordinate. Rodine, in U.S. Pat. No. 2,960,015, discloses another method of making variable transmission light filters in a two-dimensional, radially symmetric configuration in which filter transmissivity varies with radial distance from the center of a circular pattern.
Kato et al., in U.S. Pat. No. 4,253,765, disclose a multi-wavelength spectrophotometer that explicitly takes account of the decrease in sensitivity of most detectors at the lower end of the visible range of wavelengths by dividing this range into sub-ranges, each of which is scanned over different integration times. Kato et al. use a diffraction grating to provide the division of incident light into wavelength sub-ranges.
In U.S. Pat. No. 4,566,797, Kaffka et al. disclose use of a plurality of narrow band radiation emitting diodes, each of which emits almost-monochromatic light with differing central wavelengths. This provides a small, finite number of discrete wavelengths for use with a spectrophotometer, and these wavelengths cannot be varied or used to provide a continuous range of wavelengths.
Hopkins, in U.S. Pat. No. 4,746,793, disclose use of a mask to shade one or more of a plurality of photodiodes from light of undesired frequencies, in order to avoid overloading a photodiode by receipt of intense light produced by a strong spectral band in the light source. Light corresponding to the intense portion of the spectrum produced by the light source is masked off, and light from the remaining wavelength components is received in a normal manner for use in a spectrophotometer.
A two-dimensional color filter array, using red, green and blue filters in a predetermined pattern, is disclosed in U.S. Pat. No. 4,764,670, issued to Pace et al. The filter dyes are effectively positioned directly on the light sensors, and each dye is unavoidably broad band. This arrangement would not be suitable for spectrophotometry because it cannot provide a variable set of narrow wavelength bands.
U.S. Pat. No. 4,795,256, issued to Krause et al., discloses use of a first beam splitter to produce a first monochromatic light beam and a second beam splitter to produce a second monochromatic light beam, with a different central wavelength, from the remainder of the light beam. Although one could continue this to produce a small number of different wavelength light beams, the overall apparatus would be complex and large and would produce only a discrete set of fixed wavelengths, not a continuously variable set of such wavelengths.
U.S. Pat. No. 4,797,000, issued to Curtis, discloses a comparative colorimeter that compares the color densities of two liquid samples and provides a measure of the degree of difference of the two samples. The apparatus uses a differential amplifier circuit that receives electrical signals produced by receipt of light transmitted through each of the samples at a pair of photodetectors. The currents produced at the photodetectors are preferably logarithmically converted to voltage signals in order to increase the dynamic range of the apparatus. The photodetector sensors appear to use conventional discrete filter-diffuser combinations to analyze the transmitted light in each of a small number of wavelength subranges.
Several U.S. patents disclose use of a Fabry-Perot interferometer arrangement in which one of the two parallel reflecting surfaces thereof can be displaced by a controllable amount in order to vary the distance between the two reflecting surfaces and thereby vary the wavelength(s) at which maximum transmission occurs from the interferometer. These patents include U.S. Pat. No. 3,387,531 issued to Hesse, U.S. Pat. No. 3,635,562 issued to Catherin, U.S. Pat. No. 4,318,616 issued to Chamran et al., U.S. Pat. No. 4,572,669 issued to James et al., U.S. Pat. No. 4,738,527 issued to McBrien, and U.S. Pat. No. 4,825,262 issued to Mallinson.
Fein et al. disclose an optical radiation translating device in U.S. Pat. No. 3,498,693. The Fein et al. apparatus in one embodiment (FIG. 3) uses two spaced apart planar reflectors of light that are inclined at a non-zero angle relative to one another, with a wedge-shaped dielectric material occupying the volume between the reflectors. Light is transmitted through the wedge-shaped filter, requiring constructive interference of the light waves, only at positions along the device where the one-way optical path length of the light beam through the dielectric material is an integral multiple of one half the wavelength .lambda..sub.0 of the light, which is assumed to be monochromatic. The dielectric spacer material has an electrical field applied thereto, and as the magnitude of the voltage changes the positions where light beam transmissions occur are moved across the face of the device. In all embodiments discussed, the light beam is monochromatic and the number of discrete light transmission positions along the filter is finite and is equal to the number of discrete positions where the optical path length through the dielectric material is m .lambda..sub.0 /2 (m=1,2,3, . . . ). Light transmission through the device will be manifested by the appearance of one or a discrete sequence of uniformly spaced light beam spikes at intervals along the face of the device.
In U.S. Pat. No. 3,552,826, Hanes et al. disclose a variable thickness, multi-layer light reflector with a thickness h(x) that decreases exponentially with increase in a spatial coordinate x, measured in a predetermined direction in a plane of the reflector. The reflectance R of the reflector at any point x is a function of the single variable w=.lambda./h(x), where .lambda. is the wavelength of light incident on the reflector. The exponential decrease of thickness h(x) with the coordinate x is required in order to insure that .differential..sup.2 R/.differential..lambda..differential..sub.x =0 and .differential..sup.2 R/.differential.x.sup.2 =0.
Bates, in U.S. Pat. No. 3,929,398, also discloses use of a wedge-shaped interference filter to produce a line of light at a particular coordinate position x that varies with the wavelength of the incident monochromatic light. The position x of the line of light is variable and is controlled by the operator's choice of wavelength. A sequence of masks is used to selectively mask portions of the line to produce an ordered sequence of dark and light regions on the illuminated line that characterizes the light (e.g., its wavelength).
A color sensing device using a group of adjacent, non-overlapping light filters with different pass bands is disclosed by Hinoda et al. in U.S. Pat. No. 4,547,074. Each light filter consists of an interference filter with a plurality of separated wavelength pass bands plus a color filter with a sharp cutoff band that falls within one of the interference filter pass bands. The serial combination of these two filters selects a particular, fixed narrow wavelength band for transmission of light therethrough. A photodiode, positioned beneath the serially combined interference filter and color filter, receives the transmitted light and determines the relative intensities of light in each of several wavelength pass bands. Photodiode light-receiving faces may have different areas to reflect the light sensitivity of the photodiodes in different, fixed wavelength regions. A subgroup of such filters may be configured to sense the relative amount of light in each of a set of adjacent wavelength bands, to thereby provide color matching capability according to the CIE XYZ colorimetric system. The incident light is not assumed to be monochromatic, but it appears that each interference filter/color filter pair must be carefully matched to provide a narrow, fixed wavelength pass band.
U.S. Pat. No. 4,822,998, issued to Yokota et al., discloses use of an array of light sensors, each sensor being sensitive to a different wavelength range and receiving light transmitted through a light filter with a transmission wavelength band pass corresponding to the wavelength band to which the light sensor responds. In one embodiment, shown in FIG. 1 of the Yokota et al. patent, the light filter array is arranged in a double staircase configuration, with the filter thickness increasing from one plateau of constant thickness to another plateau of greater constant thickness. A first filter staircase and second filter staircase have filter thicknesses chosen to correspond to optical interference orders m=1 and m=2, respectively, according to well known optical interference relations for a Fabry-Perot etalon. 0.7 .mu.m) into two smaller wavelength ranges, the sidebands of each interference order, other than the order m=1 or m=2 that is desired, are caused to appear at wavelengths well removed from the visible spectrum and can be attenuated with simple fixed band pass ultraviolet and/or infrared filters. Low order Fabry-Perot interference bands are usually not narrow enough by themselves for most spectrophotometer applications. As FIG. 7 of the Yokota et al. patent illustrates, the full width at half maximum ("FWHM") for a low order interference band, with a central wavelength .lambda..sub.c =400 nm, is 15 nm and 9.6 nm for surface reflectivities of R=0.23 and 0.62, respectively. The FWHM would increase with increasing wavelength. These FWHM values are much too wide for many applications of such technology in colorimeters and radiometers. Increasing the reflectivity R of the surfaces of the Fabry-Perot etalon will narrow the FWHM by a modest amount, but the FWHM is still too large for some spectrophotometer applications, and the transmissivity T=1-R may already be so low that the signal-noise ratio for the photosensor signals becomes a concern. The wavelength skirts that extend beyond the FWHM wavelength region may also be too broad to allow sharp wavelength discrimination.
Many of the devices of the prior art are large and bulky and do not make full use of or analyze all wavelengths in a continuous wavelength interval of the incident light beam. The cost of these devices is usually great, due in part to the delicate optical systems used. Further, no controllable means has been disclosed for compensating, at the same time, for the non-uniform sensitivity, as a function of wavelength, of photodetector elements or for compensating for use of a non-standard light source for illumination.
What is needed is a compact apparatus that (1) efficiently disperses a light beam into a continuous interval of wavelengths and analyzes the content of the light beam throughout this interval; (2) allows shifting or augmentation of the wavelength interval to be analyzed; (3) allows flexibility in alteration of the light beam intensity distribution received by a plurality of wavelength-sensitive photodetector elements; (4) provides sharply defined, very narrow bands of light of different wavelengths at each photodetector element; and (5) allows construction of the apparatus on a single chip that is compact and rugged and has low cost.